1. Field of the Invention
The invention relates to imaging devices particularly with respect to the detection and recognition of remote objects.
2. Description of the Prior Art
Thermal imaging sensors are often used to search for and identify objects. These sensors capture the intensity of radiation from the surfaces of the objects. A fundamental limitation when using thermal intensity for search and identification is that thermal intensity gives one parameter whereas the surface orientation on a three-dimensional object is specified by two angles. Information about the two angles of surface orientation is often contained in the polarization of the thermal radiation. Polarization also gives useful information about the surface properties of the object. Man-made objects have unnaturally smooth surfaces, which in turn produces radiation with greater polarization. Natural backgrounds such as grass, trees, dirt, and sand generate radiation that is less polarized.
The polarization properties of a beam of incoherent radiation emitted or reflected from a object's surface can be completely described at a given wavelength by the four Stokes parameters, (I, Q, U, V). The first Stokes parameter I is a measure of the total intensity of radiation. The second parameter Q measures the amount of linear polarization in the horizontal direction. The third parameter U measures the amount of linear polarization at 45 degrees from the horizontal. She fourth parameter V is associated with the circular polarization.
The Stokes parameters I, Q, ani U can be transformed into percent of linear polarization P and angle of polarization plane .phi. using the relations, ##EQU1## As described later, P and .phi. are directly related to the surface orientation of the object.
Conventional methods of determining polarization from images rely on the use of a single polarizer covering the entire imaging sensor. The polarizer is rotated and a sequence of image frames are captured. An example of this method is described in "Polarization imagery," by R. Walraven in Optical engineering, vol. 20 no. 1 (1981), pp. 14-18. Walraven uses a sequence of four image frames captured with a linear polarizer oriented at 0.degree., 45.degree., 90.degree., and 135.degree.. The first three Stokes parameters can be determined at each image pixel, EQU I=1/2.multidot.(i.sub.0 +i.sub.45 +i.sub.90+i.sub.135) EQU Q=i.sub.0 -i.sub.90 EQU U=i.sub.45 -i.sub.135
where i.sub.x is the intensity measured with the polarizer oriented at x degrees.
A disadvantage to using a single polarizer is that the mechanism for rotation requires maintenance and is subject to failure during operation. Another disadvantage is that, because a large aperture polarizer is bulky, a high rotation speed is impractical, which limits how close in time the sequence of images can be captured. If the object is moving rapidly or the sensor system is vibrating, the object will be misregistered, i.e. in a different position in each image, in the sequence. A further disadvantage is that, the substrates on which polarizers are fabricated do not have plane parallel surfaces. When such substrates are rotated, the image position at the photodetectors will wander, resulting again in misregistration. Yet another disadvantage is that a sensor with a mechanism for rotating a polarizer at a high rotation speed would be massive and large and not portable.
A polarization-sensitive thermal imaging sensor which overcomes these disadvantages was disclosed by the present inventor in U.S. Pat. No. 5,416,324 (1995). This sensor is able to capture Stokes parameters I, Q, and U in a single image frame because the sensor uses an array of pixel-size polarizers in front of a two-dimensional focal plane array of photodetectors. Each pixel-size polarizer is a linear polarizer which is fixed in a rotated position relative to its nearest neighbors. Polarization data is derived from measuring the amounts of light which passes through polarizers with at least two different rotation orientations. The use of the polarizer array permits polarization data to be captured accurately, provided that the radiation from the object does not vary significantly from one pixel to its neighboring pixel. This would occur when objects extend over many pixels.
However, a disadvantage of using the polarizer array in the inventor's patent is that polarization data cannot be captured accurately when objects extend over less than one pixel, i.e. point source objects. This would be the case when attempting to detect an airplane at long range or when the detailed features on a object are important for its identification.